UV barrier film

ABSTRACT

Transparent polypropylene film comprising at least two UV-absorbing additives, a first additive being a non-aggregated inorganic material present in the film composition in an amount of from 0.1 to 5.0% by weight, and a second additive comprising an organic material selected from triazines, hindered amines, oxanilides, cyanoacrylates, benzotriazoles and/or benzophenones and present in the film in an amount of less than 2.0% by weight and wherein when both benzotriazole(s) and benzophenone(s) are present in the film, the ratio of benzotriazole(s) to benzophenone(s) is above 0.5.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/670,356, a U.S. National Phase under 35 U.S.C. §371 of InternationalApplication No. PCT/GB2008/050596, filed on Jul. 18, 2008 and publishedin English on Jan. 29, 2009, which claims foreign priority from GB0714418.1, filed on Jul. 24, 2007, each of which are incorporated hereinby reference in their entirety.

BACKGROUND

1. Field

The present invention concerns ultraviolet (“UV”) barrier films, inparticular polymeric films comprising a mixture of both inorganic andorganic UV blocking compounds.

2. Description of the Related Art

UV barrier films are well known in the art. Thus, WO-A-04/87795discloses a biaxially oriented polypropylene (BOPP) film which hasdispersed therein rigid rod like UV absorbers compatible with thepolymer such as (E,E)-1,4-diphenylbuta-1,3-diene (DPBD);(E,E)-1,6-diphenylhexa-1,3,5-triene (DPHT); and beta, beta-carotene(carotene) at typical concentrations of about 0.01% w/w. These absorberspreferentially align along one of the two orientation axes of the filmand allow anisotropy in the film to be covertly detected by thedifference in the UV fluorescence spectra of the film in along eachaxis. This provides a covert means to authenticate the film, which isuseful as a substrate for security applications.

EP-B-1004626 discloses a thin packaging film made out of a thermoplasticmaterial with anti-UV properties, characterized in that its compositionfurther comprises a combination of at least one organic compound with atleast one inorganic UV-absorbent compound, for improved barrier againstUV rays, and improved transparency. Preferably, the organic compound isa benzotriazole, and the inorganic compound is micronized zinc oxide.More preferably, the material for the film further comprises a bindercompound, for example a benzophenone, in order to improve thecompatibility between the organic component and the base-resin.

EP-A-160981 discloses a polymer composition which is used as amasterbatch comprising polyolefin and 10 to 80 wt-% nano scalar zincoxide having a particle size between 1 and 100 nm. The invention furtherrelates to a film made from this masterbatch and to a method formanufacturing said film.

U.S. Pat. No. 6,916,867 discloses polyolefin compositions which compriseas UV absorber a synergistic mixture of a) at least onehydroxybenzophenone and at least one 2-hydroxyphenylbenzotriazole withthe proviso that the polyolefin is a high density polyethylene of the“Phillips” type or a polyethylene of the metallocene type; b) at leastone hydroxybenzophenone and at least one 2-hydroxyphenyltriazine, withthe proviso that if the polyolefin is polypropylene, no polyvinylpyridinis present c) at least one hydroxybenzophenone and at least oneoxanilide; d) at least one 2-hydroxyphenylbenzotriazole and at least oneoxanilide; e) at least one 2-hydroxyphenyltriazine and at least oneoxanilide; f) at least one hydroxybenzophenone, at least one2-hydroxyphenylbenzotriazole and at least one oxanilide; g) at least onehydroxybenzophenone, at least one oxanilide and at least one2-hydroxyphenyltriazine; or h) at least one2-hydroxyphenylbenzotriazole, at least one oxanilide and at least one2-hydroxy phenyltriazine.

SUMMARY

According to embodiments of the present invention there is provided atransparent polypropylene film comprising at least two UV-absorbingadditives, a first additive being a non-aggregated inorganic materialpresent in the film composition in an amount of from 0.1% to 5.0% byweight, and a second additive comprising an organic material selectedfrom triazines, hindered amines, oxanilides, cyanoacrylates,benzotriazoles and/or benzophenones and present in the film in an amountof less than 2.0% by weight and wherein when both benzotriazole(s) andbenzophenone(s) are present in the film, the ratio of benzotriazole(s)to benzophenone(s) is greater than 0.5.

The inorganic additive is preferably selected from one or more mineraloxides such as metal oxides, for example from non-aggregated zinc and/ortitanium oxides. The mean particle size of the inorganic additive ispreferably <200 nm, more preferably <100 nm, more preferably <75 nm,still more preferably <50 nm and most preferably <40 nm. Non-aggregationof the inorganic additive can be achieved by means known in the art,such as coating, dispersion, etc.

Although it has previously been contemplated to use a combination ofinorganic and organic UV-blockers in a polymeric film, the film of theinvention realises a significant advantage in optical properties, and bycareful selection of the relative quantities of the respectiveadditives, it has been found possible to formulate the film compositionwithout the aid of a binder to solubilise the organic component.

One problem with organic UV absorbers is their tendency to bloom ormigrate to the film surface over time, causing a deterioration in theoptical properties of the film. We have found that in a polypropylenefilm the problem of migration of organic additives such as triazines,hindered amines, oxanilides, cyanoacrylates, benzotriazoles and/orbenzophenones can largely be avoided by limiting the quantity of suchadditive in the film to below 2.0%, preferably below 1.5%, morepreferably below 1.0% and most preferably below 0.75% by weight of thefilm composition.

However, at such levels of organic additive, we have found that thefilms can be insufficiently UV absorbing for certain applications.

One problem with inorganic UV absorbers is their tendency to cause ahazy appearance in the film, probably because of the particulate natureof such additives, which may have particle sizes effective to dispersevisible light, or which may agglomerate in the film to do so. We havefound that in a polypropylene film the problem of haze can largely beavoided by selected a non-aggregated additive and by limiting thequantity of such additive in the film to below 5.0%, preferably below4.5%, more preferably below 4.0% and most preferably below 3.5% byweight of the film composition. We have found for example thatsatisfactory properties can be achieved with loadings of from 0.1% byweight to 3.0% by weight active inorganic additive, for example.

We have found the stated combination of inorganic and organic UVblocking additives provides a polypropylene film which exhibitsexcellent optical properties and yet is substantially UV blocking, tothe extent that at the point of maximum transmittance of UV lightbetween 220 and 350 nm the film transmits no more that 30%, preferablyno more than 25%, more preferably no more than 20% and most preferablyno more than 15% of the UV light incident upon the film at thewavelength of the said maximum transmittance.

Moreover, we have found that satisfactory optical properties withrespect to the migration of the organic additive can be realised withoutincluding in the film a binder therefor. When both benzotriazole andbenzophenone are present in the film, we have found that thebenzophenone acts as a UV absorber in its own right, not as a binder forthe benzotriazole. When both components are present, the benzophenone isin any event present in insufficient quantity with respect to thebenzotriazole to act as a binder therefor, the ratio of benzotriazole tobenzophenone in the film composition being greater than 0.5, preferablyat least 1, more preferably at least 1.5 and most preferably at leastabout 2.

The film of the invention preferably exhibits wide angle haze (WAH) ofless than 10%, more preferably less than 8% and most preferably lessthan 6%.

The film of the invention preferably exhibits gloss of greater than 80%,more preferably greater than 85% and most preferably greater than 90%.

The film may be a multilayer structure formed by any suitable method(such as co-extrusion and/or lamination) with one or more core orsurface layers being formed as described herein.

In one embodiment of the invention the film comprises biaxially orientedpolypropylene (BOPP). The BOPP films may be prepared with substantiallybalanced physical properties, for example as can be produced usingsubstantially equal machine direction and transverse direction stretchratios, or can be unbalanced where the film is significantly moreoriented in one direction (MD or TD). Sequential stretching can be used,in which heated rollers effect stretching of the film in the machinedirection and a stenter over is thereafter used to effect stretching inthe transverse direction, or simultaneous stretching, for example usingthe so-called bubble process. The machine direction and transversedirection stretch ratios are preferably in the range of from 4:1 to10:1, and more preferably from 6:1 to 8:1.

Many suitable benzotriazoles may be contemplated for use in the presentinvention, of which 2-(2′-hydroxy-3′,5′-di-t-amylphenyl) benzotriazoleavailable under the trade name Cyasorb UV-2337 from Cytec IndustriesInc. and under the trade name Lowilite 28 from Great Lakes ChemicalCorporation and phenol,2-(5-chloro-2H-benzotriazole-2-yl)-6-(1,1-dimethylethyl)-4-methylavailable under the trade name Tinuvin 326 and2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol availableunder the trade name Tinuvin 234 from Ciba Specialty Chemicals Inc. maybe mentioned as examples.

Many suitable benzophenones may be contemplated for use in the presentinvention; of which methanone, 2-hydroxy-4-(octyloxy)-phenyl availableunder the trade name Chimassorb 81 from Ciba Specialty Chemicals Inc.and2-[4,6-bis(2,4-dimethylphenyl)-1,3,5-triazin-2-yl]-5-(octyloxy)phenolavailable under the trade name Cyasorb UV-1164 from Cytec IndustriesInc. may be mentioned as examples.

Many suitable combinations of benzotriazole(s) and benzophenone(s) maybe contemplated for use in the present invention, of which Shelfplus UV1400 available from Ciba Specialty Chemicals Inc. may be mentioned as anexample.

Commercially available materials may also comprise a blend of one ormore organic UV absorbers, together with one or more inorganic UVabsorbers, of which Shelfplus UV 1400 is also an example.

Amongst inorganic UV absorbers may be mentioned micronised metal oxidessuch as zinc and titanium oxides, and mixtures thereof. Suitable zincoxide UV additives are commercially available for example under thetrade name Bayoxide from Borchers GmbH.

Other suitable inorganic UV absorbers include a polypropylenemasterbatch containing 10% mineral oxide available under the designationUVBLOCK10 from Ampacet.

The film core or the skin layers of the film may comprise additionalmaterials such as anti-block additives, opacifiers, fillers,cross-linkers, colourants, waxes and the like.

The film may be further treated, by coroner discharge treating forexample, to improve ink receptivity of the film or of the skin layer ofthe film.

The films used in accordance with the present invention can be of avariety of thicknesses according to the application requirements. Forexample they can be from about 10 to about 240 microns thick, andpreferably from about 20 to about 60 microns thick.

In the case where the film is a multilayer film having one or more skinlayers, the skin layers preferably have a thickness of from about 0.05microns to about 2 microns, preferably from about 0.1 microns to about1.5 microns, more preferably from about 0.2 microns to about 1.25microns, most preferably from about 0.3 microns to about 0.9 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be more particularly describedwith reference to the following Examples. The UV spectra mentioned inthe Examples are shown in the Figures, in which:

FIG. 1 depicts the UV spectra of Examples 1 to 4;

FIG. 2 depicts the UV spectra of Examples 5 to 10;

FIG. 3 depicts the UV spectra of Examples 11 to 15;

FIG. 4 depicts the UV spectra of Examples 16 to 20;

FIG. 5 depicts the UV spectra of Examples 21 to 26;

FIG. 6 depicts the UV spectra of Examples 27 to 32;

FIG. 7 depicts the UV spectra of Examples 33 to 38;

FIG. 8 depicts the UV spectra of Examples 39 to 44;

FIG. 9 depicts the UV spectra of Examples 45 to 50;

FIG. 10 depicts the UV spectra of Examples 51 to 56;

FIG. 11 depicts the UV spectra of Examples 57 to 62;

FIG. 12 depicts the UV spectra of Examples 63 to 68.

DETAILED DESCRIPTION

In the following Examples the UV blocking additives which were used wereas follows:

Organic Additives:

Tinuvin 234 (member of the benzotriazole family, namely2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol from CibaSpecialty Chemicals

Inorganic Additives:

Mineral oxide in PP called UVBLOCK10 (an inorganic, non-migratoryadditive) from Ampacet Europe; active ZnO in PP; and active TiO₂ in PP.

EXAMPLES 1 TO 4

Masterbatches were prepared by compounding UV barrier additives in thepowder state with polypropylene pellets using a PRISM twin screwextruder.

Plaques of polypropylene were produced with a Rondol multilayer extruderby mixing polypropylene granules and various UV barrier additivesmasterbatch. These plaques were then stretched using a TA longstretcher.

Gloss (45° angle), Wide angle haze were measured for each sample. The UVvisible spectrum was acquired using a UV visible spectrophotometer andfilm thickness was determined to make necessary correction to take inaccount thickness variation. All UV graphs have been normalised to 25 μmthickness

Accelerated ageing was also carried out by exposing samples to a 50° C.oven for 30 days in order to assess the stability of the final product.

The results are shown in Table 1 and in FIG. 1.

TABLE 1 Gloss Wide Angle Migration/ Example Sample (%) Haze (%) Blooming1 Control 102.0 0.7 n/a (comparative) 2 +0.2% Tinuvin 234 100.0 0.6 No(comparative) 3 +0.5% Tinuvin 234 102.0 0.6 No (comparative) 4 +1%Tinuvin 234 105.0 0.6 Yes (comparative)

These results show the tendency of the organic additive to bloom to thesurface of the film as the loading of the additive is increased.

EXAMPLES 5 TO 10

The samples were made up in the manner described for Examples 1 to 4,and the results are shown in Table 2 and in FIG. 2.

TABLE 2 Gloss Wide Angle Migration/ Example Sample (%) Haze (%) Blooming5 Control 102.0 0.7 n/a (comparative) 6 +0.1% mineral oxide 100 1.1 No(comparative) 7 +0.2% mineral oxide 98 2.6 No (comparative) 8 +0.3%mineral oxide 90 3.6 No (comparative) 9 +0.4% mineral oxide 96 4.7 No(comparative) 10  +0.5% mineral oxide 85 6.1 No (comparative)

These comparative results show the tendency of a first type of inorganicadditive to affect the optical properties of the film as the loading ofthe additive is increased.

EXAMPLES 11 TO 15

The samples were made up in the manner described for Examples 1 to 4,and the results are shown in Table 3 and in FIG. 3.

TABLE 3 Gloss Wide Angle Migration/ Example Sample (%) Haze (%) Blooming11 Control 102.0 0.7 n/a (comparative) 12 +0.5% Active TiO₂ 96 2.6 No(comparative) 13 +1% Active TiO₂ 95 2.4 No (comparative) 14 +2% ActiveTiO₂ 96 4.3 No (comparative) 15 +3% Active TiO₂ 90 5.8 No (comparative)

These comparative results show the tendency of a second type ofinorganic additive to affect the optical properties of the film as theloading of the additive is increased.

EXAMPLES 16 TO 20

The samples were made up in the manner described for Examples 1 to 4,and the results are shown in Table 4 and in FIG. 4.

TABLE 4 Gloss Wide Angle Migration/ Example Sample (%) Haze (%) Blooming16 Control 102.0 0.7 n/a (comparative) 17 +0.5% Active ZnO 94 1.4 No(comparative) 18 +1% Active Zno 96 1.5 No (comparative) 19 +2% ActiveZnO 100 2.2 No (comparative) 20 +3% Active ZnO 95 3.6 No (comparative)

These comparative results show the tendency of a third type of inorganicadditive to affect the optical properties of the film as the loading ofthe additive is increased.

TABLE 5 Gloss Wide Angle Migration/ Example Sample (%) Haze (%) Blooming21 Control 102.0 0.7 n/a (comparative) 22 +0.5% Tinuvin 234 102.0 0.6 No(comparative) 23 +1% Tinuvin 234 105.0 0.6 Yes (comparative) 24 +0.2%mineral oxide 98 2.6 No (comparative) 25 +0.5% Tinuvin 234 + 95 2.8 No0.2% mineral oxide 26 +1% Tinuvin 234 + 95 2.6 Yes 0.2% mineral oxide

EXAMPLES 27 TO 32

The samples were made up in the manner described for Examples 1 to 4,and the results are shown in Table 6 and in FIG. 6.

TABLE 6 Gloss Wide Angle Migration/ Example Sample (%) Haze (%) Blooming27 Control 102.0 0.7 n/a (comparative) 28 +0.5% Tinuvin 234 102.0 0.6 No(comparative) 29 +1% Tinuvin 234 105.0 0.6 Yes (comparative) 30 +0.3%mineral oxide 90 3.6 No (comparative) 31 +0.5% Tinuvin 234 + 95 3.7 No0.3% mineral oxide 32 +1% Tinuvin 234 + 97 3.5 Yes 0.3% mineral oxide

EXAMPLES 33 TO 38

The samples were made up in the manner described for Examples 1 to 4,and the results are shown in Table 7 and in FIG. 7.

TABLE 7 Gloss Wide Angle Migration/ Example Sample (%) Haze (%) Blooming33 Control 102.0 0.7 n/a (comparative) 34 +0.5% Tinuvin 234 102.0 0.6 No(comparative) 35 +1% Tinuvin 234 105.0 0.6 Yes (comparative) 36 +0.4%mineral oxide 96 4.7 No (comparative) 37 +0.5% Tinuvin 234 + 94 4.8 No0.4% mineral oxide 38 +1% Tinuvin 234 + 93 5.4 Yes 0.4% mineral oxide

EXAMPLES 39 TO 44

The samples were made up in the manner described for Examples 1 to 4,and the results are shown in Table 8 and in FIG. 8.

TABLE 8 Gloss Wide Angle Migration/ Example Sample (%) Haze (%) Blooming39 Control 102.0 0.7 n/a (comparative) 40 +0.5% Tinuvin 234 102.0 0.6 No(comparative) 41 +1% Tinuvin 234 105.0 0.6 Yes (comparative) 42 +2%Active TiO₂ 96 4.3 No (comparative) 43 +0.5% Tinuvin 234 + 90 4.8 No 2%Active TiO₂ 44 +1% Tinuvin 234 + 91 5.2 Yes 2% Active TiO₂

EXAMPLES 45 TO 50

The samples were made up in the manner described for Examples 1 to 4,and the results are shown in Table 9 and in FIG. 9.

TABLE 9 Gloss Wide Angle Migration/ Example Sample (%) Haze (%) Blooming45 Control 102.0 0.7 n/a (comparative) 46 +0.5% Tinuvin 234 102 0.6 No(comparative) 47 +1% Tinuvin 234 105 0.6 Yes (comparative) 48 +2% ActiveZnO 100 2.2 No (comparative) 49 +0.5% Tinuvin 234 + 96 2.6 No 2% ActiveZnO 50 +1% Tinuvin 234 + 96 2.5 Yes 2% Active ZnO

EXAMPLES 51 TO 56

The samples were made up in the manner described for Examples 1 to 4,and the results are shown in Table 10 and in FIG. 10.

TABLE 10 Gloss Wide Angle Migration/ Example Sample (%) Haze (%)Blooming 51 Control 102.0 0.7 n/a (comparative) 52 +0.5% Tinuvin 234 1020.6 No (comparative) 53 +1% Tinuvin 234 105 0.6 Yes (comparative) 54+0.5% Active TiO₂ + 93 3.1 No (comparative) 1.5% Active ZnO 55 +0.5%Tinuvin 234 + 93 3.3 No 0.5% Active TiO₂ + 1.5% Active ZnO 56 +1%Tinuvin 234 + 92 3.5 yvs 0.5% Active TiO₂ + 1.5% Active ZnO

EXAMPLES 57 TO 62

A three layer polymeric tube was formed by co-extruding a core layer ofpolypropylene homopolymer with two skin layers ofpolyethylene/polypropylene/polybutylene terpolymer (a random copolymer)on opposite sides of the core layer. UV additive masterbatch containingthe selected UV additive was blended prior to extrusion with thepolypropylene homopolymer in the core. The tube was cooled andsubsequently reheated before being blown to produce a three layerbiaxially oriented film tube. The blown film tube was spliced and cut toprovide a three layer film of 23 μm thickness. The optical and UVtransmittance properties of the films were measured and the resultsshown in Table 11 and in FIG. 11. All UV graphs have been normalised to25 μm.

TABLE 11 Gloss Wide Angle Migration/ Example Sample (%) Haze (%)Blooming 57 Control 92.1 1.9 n/a (comparative) 58 +0.63% Tinuvin 23489.8 3.9 n/r 0.19% mineral oxide 59 +0.55% Tinuvin 234 + 87.7 4.7 n/r0.22% mineral oxide 60 +0.5% Tinuvin 234 + 87.1 5.5 n/r 0.25% mineraloxide 61 +1% Tinuvin 234 + 85.7 6.7 n/r 0.4% mineral oxide 62 +1%Tinuvin 234 + 85.7 8.2 n/r 0.5% mineral oxide

EXAMPLES 63 TO 68

The UV spectra shown in FIG. 12 show the transmission profiles of onefilm in accordance with the invention (0.5% Tinuvin 234+0.5% ActiveTiO₂+1.5% Active ZnO in PP) compared with five comparative Examples(three being the same respectively as comparative Examples 52, 54 and 57above, and two being different film types, namely commercially availablecellulose and cellulose acetate films respectively).

What is claimed is:
 1. A transparent polypropylene film comprising: atleast a first and a second ultraviolet (UV)-absorbing additive; whereinsaid first additive comprises a non-aggregated inorganic materialpresent in the film composition in an amount of from 0.1% to 5.0% byweight, and wherein said second additive comprises an organic materialselected from the group consisting of triazines, hindered amines,oxanilides and cyanoacrylates; wherein said second additive is presentin the film in an amount of less than 1.0% by weight, wherein theaverage particle size of the first additive is less than 100 nm.
 2. Afilm according to claim 1 wherein the first additive comprises one ormore compounds selected from the group consisting of: mineral and metaloxides.
 3. A film according to claim 2 wherein the first additivecomprises one or more compounds selected from the group consisting of:zinc and titanium oxides.
 4. A film according to claim 1 formulated inthe absence of any binder material for solubilizing the second additive.5. A film according to claim 1 wherein the first additive is present inan amount below 4.5% by weight of the film composition.
 6. A filmaccording to claim 5 wherein the first additive is present in an amountbelow 3.5% by weight of the film composition.
 7. A film according toclaim 1 wherein at the point of maximum transmittance of UV lightbetween 220 and 350 nm the film transmits no more than 30% of the UVlight incident upon the film at the wavelength of the said maximumtransmittance.
 8. A film according to claim 1 further comprisingbenzotriazole and benzophenone, wherein the ratio of said benzotriazoleto said benzophenone is at least 1:1.
 9. A film according to claim 1wherein benzophenone is absent in the film composition.
 10. A filmaccording to claim 1, wherein said film exhibits a wide angle haze (WAH)of less than 10%.
 11. A film according to claim 1, wherein said filmexhibits a gloss at a 45° angle of greater than 80%.
 12. A filmaccording to claim 1 further comprising benzotriazole and benzophenone,wherein the ratio of said benzotriazole to said benzophenone is greaterthan 0.5.